Looks like nextcloud 26-29 broke something in the handling of dynamic
groups via memberURL attribute (and keeps repopulating the group —
possibly due to paging — thereby spamming members with “An administrator
removed you from group medlemmar” mails), so we expand on the slapd via
slapo-dynlist(5) instead.

This commit also fixes an issue with the openldap module where the index
of the leftmost attribute of the DN is not necessary {0}.

Also, switch from rsa4096 to ed25519 and use a separate key for each
syncrepl.

Run as a dedicated user, not ‘postfix’.

This provides better isolation opportunity as the service doesn't need
to run as ‘vmail’ user.  We use a dedicated system user instead, and
LDAP ACLs to limit its access to the strict minimum.

The new solution is also more robust to quoting/escaping, and doesn't
depend on ‘home=/home/mail/virtual/%d/%n’ (we might use $entryUUID
instead of %d/%n at some point to make user renaming simpler).

OTOH we no longer lists users that have been removed from LDAP but still
have a mailstore lingering around.  This is fair.

This was forgotten after a092bfd947773281a23419ee0ab62358371b7166.

The following policy is now implemented:

    * users can use their SASL login name as sender address;
    * alias and/or list owners can use the address as envelope sender;
    * domain postmasters can use arbitrary sender addresses under their
      domains;
    * domain owners can use arbitrary sender addresses under their domains,
      unless it is also an existing account name;
    * for known domains without owner or postmasters, other sender addresses
      are not allowed; and
    * arbitrary sender addresses under unknown domains are allowed.